It is known in the art of solid state high power lasers such as a doped-insulator neodymium doped yttrium aluminum garnet (Nd:YAG) laser, that pumping is normally achieved by using an intense flash of white light from a flashtube (or flashlamp). The flashtube is typically excited electrically by a strong surge of electrical voltage across opposite ends of the flashtube with the tube containing an appropriate light-emitting gas.
A typical configuration of an Nd:YAG laser comprises a lasing medium (Nd:YAG) in the form of a cylindrical rod and a linear flashtube placed inside a highly reflecting elliptical chamber. The flashtube is located along one focal axis of the ellipse and the laser rod along the other focal axis of the ellipse. In this configuration, the properties of the ellipse insure that most of the radiation from the flashtube passes through the laser rod, thereby providing efficient pumping.
The optical cavity of the laser is typically formed by providing mirrors external to the elliptical chamber, one mirror being totally reflecting while the other being less then totally reflecting (e.g., transmitting about 10%) to provide an output light.
Because a large amount of heat is dissipated by the flashtube during pumping, the laser rod quickly becomes very hot. To avoid damaging the laser rod from the extreme heat, cooling is typically provided, e.g., by pumping cooling water through a jacket which surrounds the laser rod or by forcing cool air into the elliptical chamber.
Conventional operation of high power Nd:YAG lasers typically employs continuous repetitive pulsing of the pump light which provides an output beam which is also pulsed. Also a shutter is typically used to control which optical output pulses are exposed to the workpiece.
However, such continuous repetitive pump pulsing causes a thermal gradient to be introduced into the laser rod due to the heating of the entire rod by the flashtube and the cooling of the outer perimeter of the rod by the cooling process. In particular, a radially parabolic (or quasi-parabolic) temperature distribution exists in the rod along the cross-section of the rod, with the center of the rod being at a peak heated temperature and the outer radius being at the cooling temperature. Such a parabolic temperature distribution causes a corresponding variation in refractive index of the rod. This refractive index variation causes the optical path length for regions of the oscillating beams within the laser to have different optical path lengths, thereby causing portions of the internal beams (related to transverse lasing modes) to focus at different points along the rod and laser cavity. This is called a thermal lens effect (or thermal lensing) and introduces divergence into the output beam, thereby reducing the beam brightness or beam quality at the focal spot.
Consequently, this parabolic thermal gradient limits the usefulness of the laser for the processing of materials. In particular, it precludes drilling small holes or cutting fine lines in material and requires loose tolerances on the larger operations.
One way to minimize the effect of the parabolic temperature distribution, is to restrict the pumping rate to low repetition rates to allow sufficient time for the rod to completely cool between each pumping pulse, i.e., the pump time equals the thermal time constant. For typical Nd:YAG systems, the thermal time constant is approximately 2-4 seconds. Thus, when used at such low repetitions rates, the productivity and efficiency of the laser are severely limited by greatly extending the length of time required to operate on a workpiece (e.g., drill a hole or weld a joint).
One technique for improving the brightness of solid state lasers includes lasers having complex optical configurations, e.g., apertures and/or lenses, within the laser cavity to reduce the effects of beam spreading. However, this technique is very costly, is difficult to maintain in a production environment, and requires wasting much of the optical energy. Another technique is to use face-pumped rectangular "slab" crystal lasers. However, such laser crystals are very expensive (e.g., $20,000) and are hard to manufacture because the laser crystals require precise dimensional control in manufacturing, precise mounting and adequate cooling of the slab to prevent thermal gradients from distorting the slab. Still another technique used in the art to reduce thermal lensing is to drill a hole through the center of the rod and pump coolant through the hole, as well as the around the outer diameter, thereby reducing the thermal time constant and increasing the allowable repetition rate. However, this technique reduces the overall volume of the gain medium, thereby reducing the available gain and output energy of the laser.
Thus, it would be desirable to devise a scheme of operation which minimizes the quasi-parabolic temperature distribution and the resultant thermal lensing effects while not adding significant cost or complexity to the laser nor reducing the available gain or output power.